1. Field of the Invention
The present invention relates generally to an improvement in amplifier circuitry, particularly, to improved amplifier circuitry for telecommunications systems, and, more particularly, to circuitry for flexibly controlling idle current in power amplifiers within mobile or other battery-powered telephones, such as cordless or cellular phones.
2. Background and Objects of the Present Invention
The evolution of wireless communication over the past century, since Guglielmo Marconi's invention of wireless telegraphy in 1895 and his demonstration in 1897 of radio's ability to provide continuous contact with ships sailing the English Channel, has been remarkable. Since Marconi's discovery, new wireline and wireless communication methods, services and standards have been adopted by people throughout the world. This evolution has been accelerating, particularly over the last ten years, during which time the mobile radio communications industry has grown by orders of magnitude, fueled by numerous technological advances that have made portable radio equipment smaller, cheaper and more reliable. One of these advances is the increasing use of digital technology in conjunction with or instead of analog.
Within every mobile phone is circuitry to set the DC current draw from the battery to the transmission circuitry. The proper current level is essential to transmit a phone call with sufficient strength and fidelity to overcome noise in the transmission channel.
Presently, telecommunications systems, such as the Digital Advanced Mobile Telephone Service (D-AMPS), operate in both analog and digital modes with circuitry switching between the two modes of communication. That circuitry alternates between two bias set points, i.e., quiescent or idle DC current settings, one for analog mode and another for digital mode. Since the idle current of the power amplifier significantly affects many amplifier and telephone parameters, e.g., talk time, adjacent channel power (digital mode) and heat, improvements in the regulation of such idle currents in D-AMPS and other systems will lead to more efficient telephone use.
Similarly, since there are presently many different telecommunication systems, each with their own standards and technological requirements, mobile phones combining two or more of these systems will require circuitry to govern the various different levels of idle current usage.
In D-AMPS systems at present, merely setting one quiescent current for analog mode and another for digital has resulted in several problems. First, when a mobile phone need not transmit at the maximum RF power, current is wasted, i.e., the predetermined and fixed maximum current level provides for RF performance far beyond the requirement at the lower transmit power level. As discussed hereinbefore, the current level was set to insure proper operation at the maximum RF power setting. This current waste is particularly apparent in dual function (analog and digital) phones, i.e., cordless or cellular, given the increased potential for varying power level requirements. In other words, a combination cordless/cellular phone would have a higher dynamic range of required RF power levels with differing required current levels.
Further, although the generic problem of decreasing power consumption as the output requirement falls is usually solved by using a Class C bias operation, this operation is only suitable for analog FM use and not digital applications where linearity is important, e.g., in .pi./4 Digital Quadrature Phase Shift Keying modulation systems.
Another problem encountered in setting the quiescent current levels high to handle all needs, including the use of maximum RF power, is yield problems in the production of the power amplifiers. This problem is apparently the result of the manufacturer not having total control of the saturated drain current, I.sub.DSS. This would not be a problem for the high power case, but is a problem for the low power case with regard to transmit efficiency if I.sub.DSS increases. The high power case pulls more current under RF drive anyway, whereas the low power case would draw only the quiescent current which increases as I.sub.DSS increases.
Additionally, conventional power amplifiers now in use utilize a current mirror circuit, such as the one depicted in the figures of the instant application, to set the drain current in the amplifier stages. The current mirror circuit, however, is temperature dependent and sets less current when the amplifier is cold. At such lower temperatures, the amplifiers fail to sufficiently amplify the current and the circuit falls out of adjacent channel power specifications. Typically, this temperature-dependence problem is fixed by using an expensive thermistor or by setting the room temperature current at a higher level.
Although the aforedescribed temperature-dependence problem may be fixed, albeit at increased cost, by use of a thermistor, the other problems, i.e., current waste and yield, are not so easily solved and are instead typically tolerated.
It is accordingly, an object of the present invention to provide an adjustable amplifier circuit that provides variable current levels to drive transmission circuitry.
It is also an object of the present invention to reduce current waste in mobile or wireless telecommunications devices by flexibly adjusting the current level to the transmission needs.
It is an additional object of the present invention to provide a wireless telecommunications device incorporating an amplifier circuit that provides the proper current levels to transmission circuitry operating under various and different system requirements and in multimode use.
It is a further object of the present invention to provide an amplifier circuit that is less temperature-dependent.